The present invention relates to drill bits, such as bits for use in high-speed machining and/or with portable hand held drills.
A wide variety of twist drills have been developed over the years for utilization in various machining operations. The typical twist drill is characterized by a drill bit having one or more spiral groves formed in the shank of the bit. These grooves, or xe2x80x9cflutesxe2x80x9d, are useful to force lubricant to the surface being machined, which is particularly beneficial for metal machining. In addition, the flutes provide a means for moving chips from the path of the drilling operation.
One important object in the design of the drill bits is to eliminate the bunching of chips within the newly drilled hole. In many drilling operations, the bulk of the heat generated in the operation is contained within the chips. If the chips are removed, the heat generated at the point of machining is dissipated and the drill bit and part being machined stay relatively cool. On the other hand, if the chips bunch within the drilled hole, the frictional heat tends to build up, which can be damaging to the machined part, can decrease the life of the drill bit, and can even make the drilling operation more difficult.
Efficient chip removal also reduces the thrust force required to advance the drill bit through the machined material. Where the chips bunch within the drilled hole, a greater thrust force is required to continually push the bit through the material. When the drill bit finally breaks through the back side of the material, the break-through thrust tends to pull the drill bit into the workpiece, which can damage the workpiece, as well as the drill and bit itself.
It is known that for optimum chip ejection, the shorter the path that the chip must travel, the more efficient the ejection and the greater ability to prevent chip compaction. Thus, some drill bits provide spherical flutes having a lower angle helix(when measured relative to the longitudinal axis of the bit) to facilitate the chip removal. However, the lower angle helix has the disadvantage of having a lower penetration rate and lower energy efficacy in completing the drilling operation. Thus, at the other end of the scale, a higher angle helix cuts and penetrates the material much more quickly and efficiently at a given rotational speed for the drill bit.
Another important aspect of the typical rotary drill bit is the ability to maintain the bit at a desired location. A common phenomenon with a standard drill bit is that the bit will xe2x80x9cskatexe2x80x9d across the surface to be machined. Consequently, there is a need for a xe2x80x9cself centeringxe2x80x9d feature on the drill bit that allows the drill bit to be centered at the desired location more quickly and efficiently. The ability to rapidly center the drill bit is very beneficial where a number of drilling operations are to be performed in a particular workpiece.
The present invention seeks to combine the chip removal and self-centering features in a single drill bit. Thus, in one embodiment of the invention, a drill bit is provided that comprises an elongated body having a tip at one end and an engagement shank at the opposite end. A first portion of the body adjacent the tip has a first diameter and a second portion between the first portion and the engagement shank has a second diameter that is greater than the first diameter. In other words, the bit includes a reduced diameter tip that improves the centering capability of the bit.
In certain embodiments, the reduced diameter tip can have a diameter about ⅔ of the second diameter. The body can define a step between the first portion and the second portion, with the step in certain embodiments defining an included angle of less than 180xc2x0 relative to the longitudinal axis of the elongated body. In specific embodiments, the included angle is about 135xc2x0. The stepped diameter tip not only provides a self-centering capability, it also improves the starting proficiency for a newly drilled hole in a workpiece.
In a further feature of the inventive drill bit, the elongated body defines flutes on at least the second portion. Most preferably, the flutes continue across the first portion to the tip of the bit. In an important aspect of the invention, the flutes are formed at a helix angle relative to the longitudinal axis of the elongated body, where the helix angle decreases from the tip to the engagement shank. The flutes of the second portion can follow the second diameter, which can be substantially constant along the length of the second portion. The flutes can combine with the stepped diameter tip for improved drilling efficiency.
In a preferred embodiment, the helix angle starts at about 38xc2x0 adjacent the tip and decreases to a helix angle at the engagement shank of about 15xc2x0. In accordance with a feature of the invention, the helix angle can decrease in three segments along the length of the bit. This in a first segment, the helix angle decreases from about 38xc2x0 adjacent the tip to about 35xc2x0 at a first distance from the tip. This first distance can be about 0.5 inches for a typical xe2x85x9c inch bit.
In a second segment, the helix angle can decrease from about 35xc2x0 at the first distance to about 17xc2x0 at a second distance from the tip. This second distance can be about 2.0 inches. In the final segment, the helix angle decreases from about 17xc2x0 at the second distance to about 15xc2x0 at a third distance from the tip. In a preferred embodiment, the third distance can be at least 3.5 inches.
Preferably, the helix angle decreases substantially linearly over each of the three segments. The slope of the linear change can vary between segments. For instance, the slope of the helix angle change in the second segment (i.e., between the first and second distances) can be much steeper than the slope of the angle change in the other segments. The slope of the change in the second segment is preferably steep enough to provide for rapid chip removal, but not so steep as to increase the risk of chip bunching along the length of the bit. Likewise, the slope in the initial segment is preferably shallow enough to provide smooth movement of the chip away from the drilling site. Preferably, the difference in slopes between the first and second segments is not dramatic enough to risk chip bunching at the first distance.